A heightened awareness of the effects that chemical by-products have on the environment has led to more stringent governmental regulations on virtually all chemical processes. Many of these regulations are directed to limiting levels of toxics and related chemicals which can be discharged into the environment. For example, the process gases in pulp and paper manufacture include trace levels of methanol and AOX (absorbable organic halogens). These contaminants travel with the process gases and may escape into the atmosphere or into the end product if not removed prior to discharge.
There are many methods for removing gaseous contaminates and particulate matter from gas or liquid streams. One method which is finding increased interest is known as scrubbing. Scrubbing involves the mixing of a process gas or vapor with a scrubbing liquid in a vessel. The mixing of the two constituents is designed to dislodge entrained particulate matter or to remove soluble gases from the gas or vapor stream. One method for scrubbing utilizes a tray or grid through which the gas or vapor stream and the liquid stream are passed. The grid consists of a plurality of apertures. As the gas/vapor stream passes through the grid and meets with the liquid stream, ebullition or bubbling of the liquid occurs on one side of the grid. The bubbling of the two streams forms as a fluidized bed. As the process gas bubbles pass through the fluidized bed the bubbles burst creating small droplets that impact into the contaminate particles and create a large surface area for gas absorption. Alternately, if the liquid and/or gas stream is mixed with a chemical scrubbing agent, reaction and neutralization of entrained contaminate gases may occur.
Conventional scrubbing processes are designed to provide a continuous flow of gas and liquid streams. That is, the liquid and gas feed stock streams are continuously introduced into a contact vessel, and the resulting gas and liquid product streams are continuously withdrawn. Countercurrent flow of the gas and liquid streams is the most common flow pattern used in conventional scrubbers. The liquid stream is introduced at or near the top of the vessel and withdrawn at the bottom. The gas stream is fed in at or near the bottom of the vessel and withdrawn at the top. Concurrent flow type scrubbers channel both streams through the vessel in the same direction.
The grids or trays are mounted at one or more locations within the vessel so as to insure that the desired degree of contact between the gas and liquid occurs. The desired degree and duration of contact between the streams is dependent on many considerations, such as the desired quality (purity) and quantity (yield) needed. It may also be desirable to minimize the pressure drop of the gas stream through the contact or scrubbing vessel. The configuration of the apparatus is also an important feature inasmuch as the preferred vessel is small, simple and relatively easy to maintain.
U.S. Pat. No. 4,432,914, incorporated herein by reference, discloses a catenary grid scrubber which is effective in removing particulate matter and soluble gases from a process gas stream. The scrubber includes a parabolic or contoured grid designed to effectively and efficiently control the rate of gas and liquid interaction so as to maximize the capture or solubility of the entrained contaminates.
To date, scrubber effectiveness has been limited to removing particulate matter or inducing solubility of entrained gases. Alternate systems have been developed to remove insoluble contaminate gases or microscopic particulate (e.g., bacteria). One such system uses ultraviolet light to destroy or chemically convert the contaminate. Ultraviolet (UV) light systems operate by directing a high intensity UV light at a liquid steam that includes a process chemical (usually an oxidant) containing the contaminant. The properties of the UV light produce a catalytic reaction within the stream which eliminates the contaminant or converts it to a more favorable chemical composition which may be subsequently removed.
The primary deficiency with UV light systems is that they are surface area dependent. That is, the UV light cannot deeply penetrate into a liquid stream for irradiating the contaminants. UV light systems, therefore, must channel the liquid stream through thin tubes during UV light irradiation. This severely limits the quantity of irradiated product that can be produced using a UV light system. In order to increase the amount of surface area of liquid irradiated, conventional systems utilize a plurality of thin tubes, each with a suitable UV light source. UV light systems are also not effective in removing large particulate matter from gas streams.
However, as stricter government regulations are imposed on the pulp and paper, electronics and chemical processing industry, these conventional methods for cleaning gas and/or liquid streams are becoming less and less effective. For example, the government has recently proposed stricter rules on the amount and type of emissions allowed in a pulp bleaching operation. These rules propose the elimination of methanol from the pulp bleach plant scrubber exhaust. Conventional scrubbers cannot effectively remove methanol from a process stream. Conventional UV light systems are capable of removing the methanol from a liquid stream but only on a small scale.
Criticism has also developed over the use of chlorine based biocides in cooling tower water disinfection and algae control. In order to effectively disinfect water, conventional scrubbing devices would require the use of non-chlorine additives. UV light systems can be utilized but, due to the small surface area required for effectively catalyzing the contaminates, the amount of product stream cleansed is limited.
A need therefore exists for an improved system that effectively and efficiently removes particulate matter and/or soluble gaseous matter from a process stream, while producing a large liquid surface area that can be irradiated with UV light for the destruction of the absorbed organic compounds.